This invention relates to batteries with rust-preventive structures primarily incorporated in electric-powered vehicles or in equipment used where moisture is likely to accumulate on the batteries.
A single battery cell typically has an iron case with a nickel-plated surface. When moisture adheres to the surface of the metal case, the nickel plating peels off exposing the iron such that the iron may rust. Battery rusting causes poor electrical contact by increasing contact resistance, and also impedes proper operation of the safety valve. To alleviate these problems, battery packs used outdoors have a plurality of battery cells sealed within a moisture-tight case.
Because the battery cells are protected in the moisture-tight case, they can be used outdoors with confidence. However, only small capacity batteries can be employed in this structure. This is because the battery cells cannot be effectively air-cooled for applications where high currents are extracted. Since battery packs used as power supplies for applications such as electric-powered vehicles output high currents and correspondingly large amount of heat, how efficiently that heat can be given off is extremely important. When a battery heats up and its temperature rises, battery performance drops drastically. High current batteries used for applications such as electric-powered vehicles have large battery capacities. In order to reduce the charging time of high capacity batteries, charging currents are increased and effective air-cooling during charging is also important. Consequently, high current batteries, even those used outdoors, cannot be waterproofed by configuring a plurality of battery cells inside a moisture-tight case. Therefore, prior art high current batteries have the drawback that effective cooling as well as a waterproof structure are difficult to achieve.
A safety valve is provided to prevent a battery's external case from bursting. The safety valve opens when internal pressure rises abnormally. An open safety valve prevents external case rupture by exhausting gases within the battery. Provision of a safety valve makes it difficult to attain a moisture-tight seal for the battery. As discussed in Japanese Utility Model Publication No. 59-19301 issued Jun. 4, 1984 and Japanese Non-examined Utility Model Publication No. 62-59961 issued Apr. 14, 1987, this is because battery gases cannot be quickly discharged out of an opened safety valve.